Page 349 - Earth's Climate Past and Future

CHAPTER 18

Causes of Warming

over the Last

125 Years

Direct temperature measurements show that Earth’s average surface tempera-
ture has warmed by 0.7°C in the last 125 years (Chapter 17). During the past two
decades, climate scientists have been exploring how much of this warming has
been caused by natural factors and how much by the build-up of CO , CH , and
2 4
other greenhouse gases in Earth’s atmosphere as a result of human activities.
Most climate scientists now agree that tectonic, orbital, and millennial-scale
forcing do not explain this rapid warming. Although solar forcing and volcanic
forcing may have played a small role, the rapid warming of the last 125 years has
been produced largely by humans.